A seat belt prevents an occupant of a vehicle from being thrown against the interior of the passenger compartment and other objects in the case of a vehicle crash and is desired to be free from any slack in view of the effectiveness of the seat belt to restrain the occupant in case of a crash but some slack is necessary for the comfort of the occupant under normal condition. Thus, it is advantageous to have some slack in the seat belt under normal condition and remove it upon detection of a vehicle crash. To accomplish this goal, Japanese patent laid open publication No. 60-259553 discloses a seat belt preloader device which rapidly retracts a certain length of the seat belt upon detection of a crash. U.S. Pat. No. 4,580,810 discloses an inflatable air bag which is normally stowed in a steering wheel but rapidly inflates upon detection of a vehicle crash to reduce the impact of the crash upon the occupant
An acceleration sensor or a deceleration sensor for activating safety devices of these types typically comprises a sensor mass which is subjected to an inertia force when a vehicle crash occurs, a bias spring for restraining the sensor mass against an inertia force arising from a sub-threshold level deceleration and a trigger member which holds a spring loaded firing pin or the like in an inactive state by being engaged with the sensor mass. When the sensor mass is moved by the inertia force against the biasing force of the bias spring, the trigger member is disengaged from the sensor mass and releases the firing pin which, for instance, strikes a primer of a propellant cartridge.
Such a deceleration sensor for activating an occupant restraint system and other safety devices is required to be reliable in terms of its capability to be activated when required and to stay inactive when not required to be activated. However, according conventional devices for detecting a threshold level of deceleration indicative of a vehicle crash, since the biasing force of the bias spring which determines the threshold level of deceleration in relation with the mass of the sensor mass typically applies a certain pressure to the area of contact between the trigger member and the sensor mass, causing friction therebetween which tends to make the force required to disengage the sensor mass from the trigger member somehow unpredictable and causes the threshold level to be unstable.